In self-resonant ballast circuits, a signal is fed back from a resonant LC tank to the drive circuitry of a pair of half-bridge switches. Two common classes of self-resonant circuits are those which feed back a sample of the tank inductor voltage and those which feed back a sample of the LC tank current. Ballasts of the voltage feedback class have the advantage of simple construction, requiring only a single magnetic component. Secondary windings are wound on the ferrite or other magnetic core used to construct the tank inductor to act as transformer windings, providing the inductor voltage to the drive circuits of the ballast switches. Disadvantageously, however, such voltage feedback circuits suffer from poor input power regulation as either the lamp resistance or line input voltage is changed.
Ballasts of the current feedback class exhibit much better input power regulation. In such current feedback circuits, a current transformer is inserted into the LC tank circuit to feed back a sample of the tank current to the ballast drive circuitry. The transformer is wound on a magnetic core separate from the core used by the tank inductor. Disadvantageously, however, since they require two magnetic components, current feedback ballasts suffer from increased circuit and layout complexity as well as added cost.
Accordingly, it is desirable to provide a fluorescent lamp ballast which achieves current feedback performance using a single magnetic component to function as both tank inductor and current feedback transformer.